Week 8 - Simulation of a backward facing step in OpenFOAM
AIM:
- To simulate an incompressible – laminar – viscous flow through the backward facing step geometry.
- To measure the velocity profile at 0.085 m from inlet of the geometry.
GIVEN:
CASE 1 - Simulate the flow without using any grading factor (i.e., GF = 1)
CASE 2 - Simulate the flow with grading factor of 0.2. The cells should be finer near the walls (including the step wall).
MESH SPECIFICATION
- Number of cells along the x direction (longer dimension) = 200
- Number of cells along the longer y direction = 20;
BOUNDARY CONDITION SPECIFICATION
The domain specifications are provided in the following figure
GEOMETRY CREATION:
Let us consider creating the geometry using 5 blocks as shown in the figure below
The below sketch is used for finding the vertices for each block
PROCEDURE:
- Open the terminal in ubuntu by using Ctrl +Alt +T in the keyboard.
- We are going to simulate our problem in OpenFoam by using the tutorial file present in OpenFOAM.
- Type tut in the terminal to open the tutorials directory and type ls to list the files present inside the directory.
- As we are solving for incompressible flow we use the cd command to open the incompressible directory and ls to list the files inside the directory.
- The icoFoam solver is selected here because we are solving for laminar viscous flow that is present inside the incompressible directory.
- As per the requirement of geometry we are choosing the cavity folder.
- As we cannot make changes to tutorials directly we are making a copy of cavity directory to FOAM_RUN folder where we can makes changes to the problem.
- Inside the cavity folder we have initial folder, constsnt folder and the system folder.
- We are opening the system folder to make changes to the block mesh.
- The blockmeshdict file is opened to make changes . as it is a script file we use gedit command to edit the script. After making the changes the file is saved.
- To specify the simulation time the controldict script file is opened and made changesto it.
- After this our problem is all set we can now proceed with blockmesh command to deploy mesh and checkmesh command to check the quality of mesh.
- To run the simulation the icofoam solver name is just typed and the simulation will start running.
- The paraFoam command is used to view results in paraview to visualize the results and the variations.
CASE 1: simulation for the grading factor 1
BlockMeshDict:
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
convertToMeters 1;
vertices
(
(0 0 0) //0
(0.08 0 0) //1
(0.08 0.005 0) //2
(0 0.005 0) //3
(0.08 0.01 0) //4
(0 0.01 0) //5
(0.20 0.01 0) //6
(0.20 0.005 0) //7
(0.20 0 0) //8
(0.20 -0.01 0) //9
(0.08 -0.01 0) //10
(0 0 0.001) //11
(0.08 0 0.001) //12
(0.08 0.005 0.001) //13
(0 0.005 0.001) //14
(0.08 0.01 0.001) //15
(0 0.01 0.001) //16
(0.20 0.01 0.001) //17
(0.20 0.005 0.001) //18
(0.20 0 0.001) //19
(0.20 -0.01 0.001) //20
(0.08 -0.01 0.001) //21
);
blocks
(
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 1 1)
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 1 1)
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (1 1 1)
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (1 1 1)
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (1 1 1)
);
edges
(
);
boundary
(
inlet
{
type patch;
faces
(
(0 11 14 3)
(3 14 16 5)
);
}
outlet
{
type patch;
faces
(
(9 8 19 20)
(8 7 18 19)
(7 6 17 18)
);
}
frontAndBack
{
type empty;
faces
(
(0 3 2 1)
(3 5 4 2)
(2 4 6 7)
(1 2 7 8)
(10 1 8 9)
(11 12 13 14)
(14 13 15 16)
(13 18 17 15)
(12 19 18 13)
(21 20 19 12)
);
}
noslipwalls
{
type wall;
faces
(
(5 16 15 4)
(4 15 17 6)
(0 1 12 11)
(10 21 12 1)
(10 9 20 21)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //- The vertices are defined in the BlockMeshDict file by making changes to the existing file.
- Here the model is splitted into five blocks having eight vertices each.
- The number of cells in x and y direction is also should be defined.
- The grading factor is defined as 1 in this case.
ControlDict:
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "system";
object controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
application icoFoam;
startFrom startTime;
startTime 0;
stopAt endTime;
endTime 0.5;
deltaT 0.0001;
writeControl timeStep;
writeInterval 20;
purgeWrite 0;
writeFormat ascii;
writePrecision 6;
writeCompression off;
timeFormat general;
timePrecision 6;
runTimeModifiable true;
// ************************************************************************* //- This script has the numerical schemes used for the simulation.
- The start and end time are defined in the code.
INITIAL SETUP:
FOR PRESSURE:
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class volScalarField;
object p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
dimensions [0 2 -2 0 0 0 0];
internalField uniform 0;
boundaryField
{
inlet
{
type zeroGradient;
}
outlet
{
type fixedValue;
value uniform 0;
}
frontAndBack
{
type empty;
}
noslipwalls
{
type zeroGradient;
}
}
// ************************************************************************* //FOR VELOCITY:
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class volVectorField;
object U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
dimensions [0 1 -1 0 0 0 0];
internalField uniform (0.15 0 0);
boundaryField
{
inlet
{
type fixedValue;
value uniform (0.15 0 0);
}
outlet
{
type zeroGradient;
}
noslipwalls
{
type noSlip;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //RESULTS:
MESH:
As the grading factor is 1 the mesh will be of same size and even throughout the model.
VELOCITY:
PRESSURE:
VELOCITY PROFILE:
The velocity profile is shown at x = 0.085m.
PRESSURE PROFILE:
The pressure profile is shown at x = 0.085m.
CASE2: simulation for grading factor of 0.2
BlockMeshDict:
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
convertToMeters 1;
vertices
(
(0 0 0) //0
(0.08 0 0) //1
(0.08 0.005 0) //2
(0 0.005 0) //3
(0.08 0.01 0) //4
(0 0.01 0) //5
(0.20 0.01 0) //6
(0.20 0.005 0) //7
(0.20 0 0) //8
(0.20 -0.01 0) //9
(0.08 -0.01 0) //10
(0 0 0.001) //11
(0.08 0 0.001) //12
(0.08 0.005 0.001) //13
(0 0.005 0.001) //14
(0.08 0.01 0.001) //15
(0 0.01 0.001) //16
(0.20 0.01 0.001) //17
(0.20 0.005 0.001) //18
(0.20 0 0.001) //19
(0.20 -0.01 0.001) //20
(0.08 -0.01 0.001) //21
);
blocks
(
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 5 1)
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 0.2 1)
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (5 0.2 1)
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (5 5 1)
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (5 5 1)
);
edges
(
);
boundary
(
inlet
{
type patch;
faces
(
(0 11 14 3)
(3 14 16 5)
);
}
outlet
{
type patch;
faces
(
(9 8 19 20)
(8 7 18 19)
(7 6 17 18)
);
}
frontAndBack
{
type empty;
faces
(
(0 3 2 1)
(3 5 4 2)
(2 4 6 7)
(1 2 7 8)
(10 1 8 9)
(11 12 13 14)
(14 13 15 16)
(13 18 17 15)
(12 19 18 13)
(21 20 19 12)
);
}
noslipwalls
{
type wall;
faces
(
(5 16 15 4)
(4 15 17 6)
(0 1 12 11)
(10 21 12 1)
(10 9 20 21)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //- The vertices are defined in the BlockMeshDict file by making changes to the existing file.
- Here the model is splitted into five blocks having eight vertices each.
- The number of cells in x and y direction is also should be defined.
- The grading factor is defined as 0.2 in this case to vary the no of grids to make easy capturing of the values.
ControlDict:
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "system";
object controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
application icoFoam;
startFrom startTime;
startTime 0;
stopAt endTime;
endTime 0.5;
deltaT 0.0001;
writeControl timeStep;
writeInterval 20;
purgeWrite 0;
writeFormat ascii;
writePrecision 6;
writeCompression off;
timeFormat general;
timePrecision 6;
runTimeModifiable true;
// ************************************************************************* //- This script has the numerical schemes used for the simulation.
- The start and end time are defined in the code.
RESULTS:
MESH:
As the grading factor is 0.2 the mesh will not be of same size and even throughout the model.
VELOCITY:
PRESSURE:
VELOCITY PROFILE:
The velocity profile is shown at x = 0.085m.
PRESSURE PROFILE:
The pressure profile is shown at x = 0.085m.
CONCLUSION:
- If we vary the mesh quality and the grading factor it will have a straight impact on the velocity and pressure.
- The lesser grading factor gives finer mesh and the high grading factor gives larger mesh.